1. Technical Field
The present invention relates to a transfer apparatus and an image forming apparatus which transfer an image carried by a transfer belt onto a transfer material by using a transfer belt having an elastic layer and a transfer roller having an elastic layer.
2. Related Art
In an existing electrophotographic type image forming apparatus using a liquid developer, there is a problem that transfer efficiency of an image carried by a transfer belt onto a transfer material such as transfer paper is not good. In particular, when an image which is carried by the transfer belt and on which a plurality of colors are superimposed is transferred onto the transfer material, deterioration in transfer efficiency becomes a problem.
Then, an image forming apparatus in which a long transfer nip is used for transferring has been proposed. Japanese Patent Application Publication No. 2001-166611 discloses such a nip as an example. The long transfer nip includes a pressure-contact nip which presses a transfer roller against a transfer belt and a winding nip which winds the transfer belt over the transfer roller and which is formed so as to be extremely longer than the pressure-contact nip. In the image forming apparatus, a transfer material contacts with an image with pressure on the transfer belt at the long transfer nip while a time during which a transfer bias is applied is made longer. This makes it possible to increase transfer efficiency.
However, if the long transfer nip is used and if the winding nip is long, an image transferred onto the transfer material is dragged to cause deviation at the time of transferring in some cases. The deviation of the image is considered to be caused by change in belt surface speed at the long transfer nip. In general, a surface layer portion of the transfer roller is formed to be softer than a backup roller of the transfer roller. Therefore, a surface layer portion of the transfer roller is concaved with respect to the circular circumference of the transfer roller at the pressure-contact nip and a surface of the transfer belt which carries an image is bent in the extension direction along the concave portion. On the other hand, a surface of the transfer belt is bent in the compression direction along an outer circumferential surface of the transfer roller at the winding nip. Thus, as the bending directions of the surface of the transfer belt differ at the transfer nip, the surface speed of the transfer belt changes between the pressure-contact nip and the winding nip. Then, as the winding nip is made longer, the change in the surface speed of the transfer belt is larger and image deviation tends to be caused.
An elastic layer of the transfer belt is compressed by a pressure load on the pressure-contact nip. Therefore, a net length of the belt is made short at the pressure-contact nip. On the other hand, the elastic layer of the transfer belt is hardly deformed because the pressure load is not applied to the transfer belt at the winding nip. Therefore, a difference in the surface speed of the transfer belt is caused between the pressure-contact nip and the winding nip.
Further, both of a contact area between the transfer belt and the transfer roller and a contact area between the transfer belt and the transfer material are increased by using the long transfer nip. Therefore, the transfer belt easily meanders due to a difference in the pressure load in the direction orthogonal to or substantially orthogonal to the transfer material movement direction, a position of the transfer material at the transfer nip (position different from a predetermined position), or a posture (skew) of the transfer material.